Propylene oxidation in the presence of rhodium metal

ABSTRACT

PROPYLENE IS OXIDIZED WITH OXYGEN TO ACROLEIN,ACETONE, ETC. BY OXIDATION IN THE CONTACT PRESENCE OF RHODIUM METAL.

United States Patent Ori ice 3,032,833 Patented Jan. 4, 1972 3,632,833 PROPYJLENE OXIDATION IN THE PRESENCE F RHODIUM METAL Noel W. Cant and William K. Hall, Pittsburgh, Pa., as-

signors to Gulf Research dz Development Company, Pittsburgh, Pa. No Drawing. Filed Nov. 25, 1968, Ser. No. 778,810 Int. Cl. C070 45/04 US. Cl. 260--604 R 9 Claims ABSTRACT OF THE DISCLQSURE Propylene is oxidized with oxygen to acrolein, acetone, etc. by oxidation in the contact presence of rhodium metal.

This invention relates to a process for oxidizing propylene to obtain a useful mixture of organic compounds comprising acrolein, acetone and acetic acid.

In addition to the above desired compounds, by-products such as carbon dioxide and water are also formed. It was believed that the partial oxidation of propylene would yield primarily organic acids containing three carbon atoms, i.e. propionic acid. Quite unexpectedly, it has been found that the main product of the oxidation reaction is acrolein so long as the oxidation reaction is carried out in the presence of rhodium metal.

In accordance with the invention, therefore, propylene is oxidized to a product comprising acrolein by reacting propylene with a gas containing free molecular oxygen in the contact presence of rhodium metal.

In order to obtain good selectivity to the desired oxygenated compounds it is important that the propylene together with a gas containing free molecular oxygen be contacted with the rhodium metal either supported or unsupported at a temperature within the range of about 100 to about 300 0., and preferably within the range of about 120 to about 220 C.

Since the reaction is exothermic, means must be provided to control the temperature of the reaction within the limits defined above. Below the lower temperature limits defined above the reaction rate becomes too low to be economically feasible, whereas at temperatures above the upper limits, the yield of desired oxygenated compounds decreases with the concurrent production of excessive amounts of water and carbon dioxide. The temperature can be controlled by any suitable means, and one method of at least partially controlling the temperature is to dilute the rhodium metal by distending it on a suitable support material such as silica, magnesia, alumina, thoria or mixtures thereof. The rhodium metal can, of course, be used unsupported, and when this is done the metal can be in any suitable form, such as sponge form. When the rhodium is distended on a support, the surface area of the support is not critical and can suitably be between 0.1 and 600 square meters per gram. In addition to the above named supports, materials such as carbon, kieselguhr, pumice, the natural clays, mullite and Alundum are also suitable. When the rhodium metal is supported, suitable amounts of rhodium metal are from 0.2 to 30 weight percent of the total catalyst with preferred amounts from one to ten weight percent and the more preferred amounts from one to five weight percent of the total catalyst.

The method of preparing the Supported or unsupported catalysts is not critical. Suitable methods of preparing the supported catalysts, for example, include the method of incipient wetness using aqueous solutions of suitable rhodium salts such as Rh(NO '2H O followed by drying and reduction in hydrogen to obtain the metal.

Another method of controlling the temperature is to dilute the propylene-oxygen mixture with an inert gas such as nitrogen or helium. Yet another method of controlling the reaction temperature is to pass the admixture of propylene and oxygen over the rhodium metal catalyst at very high space velocities. Suitable gaseous space velocities are within the range of about one to about 2000 volumes of propylene measured at standard temperature and pressure per volume of catalyst per hour, and the preferred space velocities are from about five to about 200.

A total operating pressure of about one atmosphere is the desired operating pressure. Higher or lower pressures can be used; for example, a total pressure of from 0.5 to 15 atmospheres or more can suitable be employed.

The ratio of the partial pressure of oxygen to the partial pressure of propylene can suitably be between 0.2 and 50 and is preferably between one and five. The partial pressure of propylene should be at least 0.05 p.s.i.a. and is preferably from 0.1 to 1.0 p.s.i.a. when the total pressure is atmospheric (14 p.s.i.a.). Correspondingly higher partial pressures of propylene would be employed at correspondingly higher total operating pressures.

The propylene is oxidized in the presence of a gas containing free molecular oxygen. Pure oxygen can be used, but this creates problems of temperature control as noted above. It is preferred that the free molecular oxygen be diluted with an inert gas such as nitrogen or helium. The volume percent of free molecular oxygen in the gas containing it can suitably be between one and and is preferably between one and 20. When propylene is mixed with this gas containing free molecular oxygen the partial pressure of oxygen is usually between 0.5 and 10 p.s.i.a. and is preferably between one and four p.s.i.a. If the partial pressure of oxygen is below about 0.5 pound per square inch absolute or above about four p.s.i.a., the selectivity to the desired oxygenated products decreases, whereas between about one and four pounds per square inch absolute selectivity to the desired oxygenated compounds remains substantially constant.

The invention will be further described with reference to the following experimental work. In all of the examples to follow the following procedure was employed. A single pass flow system was used wherein a feed mixture of propylene, oxygen and helium was passed through a bed (approximately two parts by volume of catalyst) of a supported rhodium catalyst at a given temperature between and 220 C. at a flow rate of between 2400 and 3000 volumes of total feed per hour. The contact time was about 2.3 seconds and the space velocity based on the total feed was about 1500 volumes of feed per volume of catalyst per hour. The propylene space velocities for any particular run can be calculated by multiplying 1500 by the ratio of the propylene partial pressure in millimeters to the total pressure of about 740 millimeters. The reaction products were cooled to -80 C. by indirect cooling to condense the reaction product which was mostly acrolein with minor amounts of acetone, acetic acid, propionic and acrylic acids. Water and CO were formed as by-products.

EXAMPLE 1 In the run for this example, the feed mixture was passed through a bed of alpha-alumina supported rhodium catalyst containing 1.5 weight percent rhodium. The results of this run are summarized in Table I below.

EXAMPLE 2 In the run for this example, the feed mixture was passed through a bed of a silica supported rhodium catalyst containing five weight percent rhodium. The silica support was a commercial Cab-O-Sil material obtained from the Cabot Company. The catalyst was prepared by the method of incipient wetness by contacting the Cab-O-Sil with an aqueous solution of an appropriate amount of a rhodium salt, i.e. Rh(NO -2H O, drying the material and reducing with H at 300 C. to convert the salt to metallic rhodium. The results of this run are summarized in Table I below.

Referring to Table I below, the percent selectivity to the useful oxygenated products was about Vs higher for the alpha-alumina supported rhodium catalyst, i.e. 29.4 percent v. 20.3 percent, but at a lower partial pressure of propylene. In both examples about 85 percent of the liquid organic oxygenated products was a mixture of acrolein and acetone. Small amounts of acetic acid and very small amounts of acetaldehyde, acrylic and propionic acids were also found in the product.

A series of runs was made by passing a feed mixture containing varying amounts of oxygen, propylene and helium over the catalyst of Example 1 at varying conditions to determine the effect of changes in temperature and feed composition on reaction rate and selectivity to the production of acetic acid, acetone and acrolein. The 20 results of this series of runs is given on Table 11 below.

ABLE I oxygenated products appears to decrease also at the higher and lower oxygen partial pressures.

Obviously, many modifications and variations of the invention as hereinabove can be made Without departing from the spirit and the scope thereof, and such modifications and variations are intended to be included within the scope of this invention.

We claim:

1. A process for producing a product comprising acrolein, which process comprises reacting propylene with a gas containing free molecular oxygen in the contact presence of a catalyst consisting essentially of rhodium metal as the active actalytic component under reaction conditions including a temperature from 100 to 300 C. and a gaseous space velocity of from about one to about 2000 volumes of propylene per volume of catalyst per hour measured at standard temperature and pressure and wherein the ratio of the partial pressure of oxygen to the partial pressure of propylene is from 0.2 to 50.

2. The process of claim 1 wherein the temperature in [Products of propylene oxidation over supported rhodium at a total pressure of about 740 mm.]

Percent selectivity 2 to- Oxy- Pro 3 1- Oxida- Acrylic Acrogen zine Temtion 1 rate plus lern Weight presprespcravolumes, D Q- P percent sure sure ture C(lHa/ Acetic piouic acc- 1 3 Ex. No. Metal Support metal (111111.) (mm.) C) min. acid acid tone Tota 1 Rh lat-A1203 1.5 42 12 184 0.08 4 0.4 25 29.4 2 Rh SiOz 5.0 59 21 183 0.12 2.8 0.5 17 20.3

1 The oxidation rate is defined as the rate at which propylene is e per minute, e.g. if the propylene is passed over the catalyst at two 1.8 votlumes per minute is recovered unchanged, then the oxidation r minu e product.

3 Percent selectivity to useful oxygenated products.

TABLE II onverted to all products in v lumes (STP) volumes per minute and it 18 found that ate is 2.0 minus 1.8, or 0.2 volume per 2 The percent selectivity is defined as the percent of the propylene oxidized which is converted to the given ess variables] Pro- Oxida- Percent selectivity 2 to Temper- Oxygen pylenc tion rate) ature pressure pressure vols./ Acetic Acro- 3 (mm) min. acid Acetone lein Total 1 2 See footnotes Table I. 3 Selectivity to useful products.

Referring to Table II, it can be seen that the selectivity to the production of useful oxygenated products decreases slightly in going from 184 to 191 C. (compare Examples 6 and 7) but that the selectivity is rather constant through the temperature range of 161-1 84 C. (compare Examples 3-6). The percent selectivity to the production of acrolein appears to be best at intermediate propylene partial Pressures (compare Examples 8-13). The rate of oxidation appears to increase as the propylene pressure increases, which can be seen by a comparison of Examples 8l3. The rate of oxidation appears to decrease with in creasing oxygen partial pressure as can be seen by a the reaction zone is maintained in the range of about 120 to about 220 C.

3. A process according to claim 1 wherein the partial pressure of oxygen is within the range of about one to about four pounds per square inch absolute.

4. The process of claim 1 wherein the rhodium metal is deposited on a support.

5. A process according to claim 4 wherein the reaction occurs at a temperature from about 120 to about 220 C., and the amount of rhodium is between 0.2 and 30 Weight percent of the total catalyst.

6. A process according to claim 5 wherein the support is alpha-alumina.

7. A process according to claim 5 wherein the support comparison of Examples 14-18, while the selectivity to is silica.

8. A process for producing a product comprising acrolein, which process comprises reacting propylene with a gas containing free molecular oxygen in the contact presence of a catalyst consisting of rhodium metal under reaction conditions including a temperature from 100 to 300 C. and a gaseous space velocity of from about one to about 2000 volumes of propylene per volume of catalyst per hour measured at standard temperature and pressure and wherein the ratio of the partial pressure of oxygen to the partial pressure of propylene is from 0.2 to 50.

9. A process according to claim 4 wherein the support consists of carbon, silica, magnesia, alumina, thoria or mixtures thereof.

References Cited UNITED STATES PATENTS 3,428,686 2/1969 Thomas 260-604 OTHER REFERENCES Patterson et al., Journal of Catalysis, vol. 2, pp.

US. Cl. X.R. 

